Processing order in short-term memory is spatially biased in children

When processing serial information, adults tend to map elements of a sequence onto a mental horizontal line, following the direction of their reading and writing system. For example, in a Western population, the beginning of a series is associated with the left-hand side of the mental line, while its end is preferentially associated with the right. To complete the few studies that have investigated the cultural vs. innate determinants of such a spatial bias, the current study used a rotation task of unlabeled serial information. Experiment 1 measured the presence of a left-oriented bias in children from toddlers to grade 5 (aged 2 to 12 years old) and in adults. Results only showed a bias in adults. Experiment 2 was designed to avoid potential confounds identified in Experiment 1, and Experiment 3 provided more explicit information to participants, but the results still did not show a clear left-oriented bias in children. By controlling the mental rotation of our material in Experiment 4, we finally observed a global left-to-right bias across age groups, in particular between the $2^{\mathrm{nd}}$ grade (7 years old) and the $4^{\mathrm{th}}$ grade (9 years old), and in adults. Overall, this study shows that spatial-ordinal associations observed in adults can also be observed in children as early as primary school. In addition, a weaker bias was observed in younger children, which suggests that the effect could begin to emerge in preschool (4 years old) before being reinforced later with expertise in literacy.